Application of the model-free approach to low molecular weight systems with hindered internal rotation: cinnamoylmesitylene

We investigated the molecular dynamics of the molecule of cinnamoylmesitylene, a substituted chalcone. Known rotation barriers for the OC‐4—C‐3C‐2 bond of substituted chalcones are in the range of values accessible to modern NMR techniques. The internal rotation about the C‐3—C‐4 bond is found to be fast relative to the complete lineshape analysis (CLSA) time‐scale. To determine the activation parameters of overall and internal motions of the molecule, the Lipari–Szabo model‐free analysis of the relaxation times and heteronuclear NOE data was used instead. Simultaneous analysis of both heteronuclear spin–lattice relaxation times and NOE data for the two carbon atoms C‐2 and C‐7 in the OC‐4—C‐3C‐2 and mesitylene fragments at different temperatures was performed. The correlation times and activation energies of overall and internal motions and the generalized order parameter, which are measures of the molecular mobility, were thus determined. The standard enthalpies of activation, ΔH≠, calculated from the experimental data for C‐2 and C‐7, are 5.6 and 6.6 kcal mol−1, respectively. Theoretical estimates of the barriers to internal rotations by ab initio MO calculations were made to verify the experimental results. The agreement between the NMR and theoretical results was good. Copyright © 2003 John Wiley & Sons, Ltd.